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"US Special Envoy Lenderking's discussions will focus on ensuring the regular and unimpeded delivery of commodities and humanitarian assistance throughout Yemen, promoting a lasting ceasefire, and transitioning the parties to a political process," the department said in a statement.

Lenderking will go to Saudi Arabia and Oman on Thursday, where he will hold meetings with senior government officials, along with UN Special Envoy for Yemen Martin Griffiths.

"The US Special Envoy will build on the international consensus to halt the Houthi offensive on Marib, which only worsens the humanitarian crisis threatening the Yemeni people," the statement concluded.

Iran 'threat'

As the US representative travels to Saudi Arabia and Oman, military experts are concerned that Iran is a "significant" threat to the peace process.

"Unfortunately all of this is working to very strong effects as we see more and more attacks on the Kingdom of Saudi Arabia - and potentially other countries - more accuracy and more lethality. So this is a great concern to us," Lenderking said earlier this week, during a House of Representatives Foreign Affairs Committee hearing.

"Iran's support to the Houthis is quite significant, and it's lethal. We would welcome Iran playing a constructive role, if they are willing to do so. We have not seen any indication of that."

A spokesman for Iran's mission to the United Nations in New York at the time said Lenderking's claims were unsubstantiated.

"In contrast, the US has been providing the deadliest weapons to those who are using them to kill innocent men, women and children on a daily basis," the spokesman said, referring to the US' role in providing weapons to members of the Saudi-led campaign against the Houthis.

In February the Houthi rebels renewed their offensive on the oil-rich province of Marib - the last stronghold held by the internationally-recognised government in the north of Yemen.

Political analysts say that the key to ending the violence in Yemen lies in putting an end to the conflict in Marib.

Losing Marib would be "the final bullet in the head of the internationally recognised government", said Abdulghani al-Iryani, a senior researcher at the Sanaa Center for Strategic Studies.

"It will set the stage for the dismemberment of the Yemeni state. You're looking at a generation of instability and humanitarian crisis. You also will look at a free-for-all theatre for regional meddling."

The escalating conflict around Marib coincides with major changes in US policy toward the war.

Former US President Donald Trump's administration had declared the Houthis a "foreign terrorist organisation", following a campaign by Saudi Arabia supporting the move.

Biden rescinded the Houthi terrorist designation after entering office.

He also announced the US would halt support for Saudi Arabia's offensive combat operations in Yemen, saying: "This war has to end."

Taking Marib, or otherwise cutting it off, would represent a major prize for the Houthis. It is home to oil and gas fields that international firms including Exxon Mobil Corp and Total SA have interests in.